Piezoelectric transducers using micro-dome arrays

ABSTRACT

An ultrasonic piezoelectric transducer device includes a transducer array consisting of an array of vibrating elements, and a base to which the array of vibrating elements in the transducer array are attached. The base include integrated electrical interconnects for carrying driving signals and sensed signals between the vibrating elements and an external control circuit. The base can be an ASIC wafer that includes integrated circuitry for controlling the driving and processing the sensed signals. The interconnects and control circuits in the base fit substantially within an area below the array of multiple vibrating elements.

CLAIM OF PRIORITY

This application claims priority to U.S. Patent Application Ser. No.61/443,042, filed on Feb. 15, 2011, the entire contents of which arehereby incorporated by reference.

TECHNICAL FIELD

This specification relates to piezoelectric transducers.

BACKGROUND

A piezoelectric transducer includes a piezoelectric element capable ofconverting electrical energy into mechanical energy (e.g., sound orultrasound energy), and vice versa. Thus, a piezoelectric transducer canserve both as a transmitter of mechanical energy and a sensor ofimpinging mechanical energy.

An ultrasonic piezoelectric transducer device can include apiezoelectric vibrating element that vibrates at a high frequency inresponse to a time-varying driving voltage, and generates a highfrequency pressure wave in a propagation medium (e.g., air, water, ortissue) in contact with an exposed outer surface of the vibratingelement. This high frequency pressure wave can propagate into othermedia. The same vibrating element can also receive reflected pressurewaves from the propagation media, and converts the received pressurewaves into electrical signals. The electrical signals can be processedin conjunction with the driving voltage signals to obtain information onvariations of density or elastic modulus in the propagation media.

An ultrasonic piezoelectric transducer device can include an array ofpiezoelectric vibrating elements, each vibrating element can beindividually controlled with a respective driving voltage and timedelay, such that a pressure wave having a desired direction, shape, andfocus can be created in the propagation medium by the array of vibratingelements collectively, and information on the variations of density orelastic modulus in the propagation media can be more accurately andprecisely ascertained based on the reflected and/or refracted pressurewaves captured by the array of piezoelectric vibrating elements.

Conventionally, many ultrasonic transducer devices use vibratingelements formed from mechanically dicing a bulk piezoelectric materialor by injection molding a carrier material infused with piezoelectricceramic crystals.

SUMMARY

This specification describes technologies related to piezoelectrictransducers.

A piezoelectric transducer device can include one or more vibratingelements each having an inner surface suspended above and attached to abase, and an outer surface exposed to a propagation medium. The one ormore vibrating elements can vibrate in response to an appliedtime-varying driving voltage and generate a pressure wave in thepropagation medium in contact with the exposed outer surface of thevibrating elements.

Each vibrating element can include a piezoelectric element disposedbetween a drive electrode and a reference electrode. The electrode thatis farther away from the base is the outward-facing electrode of thevibrating element. The drive electrode, reference electrode, and thepiezoelectric element each have a respective flexible portion and arespective stationary portion adjoining (e.g., surrounding) the flexibleportion, and where the respective flexible portions of the driveelectrode, reference electrode, and piezoelectric element (in otherwords, the flexible portion of the entire vibrating element) are curvedconcavely or convexly relative to the base, in the absence of an appliedvoltage on the vibrating element. Alternatively, the vibrating elementcan be flat. The exposed outer surface of each vibrating element caninclude the outer surface of the outward-facing electrode of thevibrating element, or alternatively, the outer surface of a flexibleprotective coating covering the outward-facing electrode of thevibrating element.

In the transducer device, the same one or more vibrating elements canalso serve as sensing elements that, in response to varying mechanicalpressures exerted by reflected pressure waves in the propagation medium,can generate a sensed voltage across the piezoelectric element betweenthe pair of electrodes. The transducer device can alternate betweendriving and sensing modes according to a timed switch.

In various implementations, the piezoelectric transducer device caninclude a transducer array consisting of an array of vibrating elements,and a base to which the array of vibrating elements in the transducerarray are attached. In some implementations, the base can include aplurality of integrated electrical interconnects for carrying drivingsignals and sensed signals between the vibrating elements of thetransducer array and an external control circuit. In someimplementations, the base can be an ASIC wafer that includes integratedcircuitry for sending the driving voltage signals to and registering thesensed voltage signals from the array of vibrating elements. The ASICwafer can fit substantially within an area below the array of multiplevibrating elements. The array of vibrating elements in the transducerarray can share a common reference electrode on one side, and have arespective individually controllable drive electrode on the oppositeside. The common reference electrode and the individually controllabledrive electrodes of the vibrating elements can be electrically connectedto the integrated circuitry in the ASIC wafer. A vertically-orientedinterconnect can be formed between the drive electrode of each vibratingelement and the control circuitry in the ASIC wafer. Thevertically-oriented interconnects can enable a large number ofconnections to be accommodated within a small lateral area occupied bythe array of vibrating elements.

In various implementations, the piezoelectric transducer device(including the piezoelectric elements, drive electrodes, commonreference electrode of the array of vibrating elements (e.g., the curvedor flat piezoelectric vibrating elements), and the electricalconnections made to the integrated circuitry in the ASIC wafer) can befabricated using various semiconductor processing techniques (e.g.,material deposition, lithographic patterning, feature formation byetching, etc.).

Particular implementations of the subject matter described in thisspecification can be implemented so as to realize one or more of thefollowing advantages.

In some implementations, the array of vibrating elements in apiezoelectric transducer device can be created using semiconductorfabrication processes, and the dimensions and pitch of the vibratingelements in the array can be made much smaller and controlled moreprecisely than those achievable in vibrating elements formed bymechanically dicing a bulk piezoelectric material or by injectingmolding. The smaller vibrating elements and the finer pitch betweenadjacent vibrating elements can enable higher imaging resolution basedon the reflected and refracted pressure waves received by the vibratingelements. Furthermore, multiple transducer devices can be fabricated onthe same wafer using the semiconductor fabrication processes, reducingthe cost for individual transducer devices.

In various implementations, control circuitry and sensing circuitry forthe piezoelectric transducer device can be implemented in an AISC layerin the base attached to the underside of the array of curved vibratingelements. Since an ASIC layer can support a large number of internaloutput connections using a small number of external input connections,the transducer device including an integrated AISC layer for providingthe driving signals can have a smaller number of external input wires,e.g., the wire bundle to be connected to the transducer device can bethinner. By reducing the lateral area needed to accommodate the externalinput connections for the transducer device, and the overall size of thetransducer device can be reduced, which can permit the device to be usedin smaller spaces and thus in a wider variety of applications.Furthermore, much processing logic for controlling the direction, focus,shape, and/or frequency of the transmitted pressure waves can beimplemented in the AISC layer, reducing the total cost of the peripheralsupporting equipment (e.g., external driving circuits and connectioncables) of the piezoelectric transducer. In some implementations, thereduction in device size can be achieved with a base having integratedelectrical interconnects that connects to an external control circuitsor a control circuit situated at a convenient location not directlybelow the transducer array.

In addition, for a given applied voltage, a curved piezoelectric element(e.g., a piezoelectric film have a domed portion surrounded by a planarportion) has a larger displacement (e.g., 5-10 times larger) than a flatpiezoelectric membrane or a piezoelectric body (e.g., rod) of acomparable size. Therefore, by using a curved piezoelectric element ineach vibrating element of the piezoelectric transducer device, strongerpressure waves can be generated using a given driving voltage.Similarly, for a given acceptable sensed voltage level, a lowermechanical pressure is required. For example, a driving voltage of 10-20volts or less can be required for an ultrasonic transducer device madeof a micro-dome transducer array, as compared to 100-200 volts requiredfor ultrasonic transducer device formed from diced bulk piezoelectricmaterials. With a lower required driving voltage, power consumption andloss due to ohmic heating can be reduced, and excessive temperatures onthe transducer device due to ohmic heating can be also avoided. This canalso permit the device to be used in a wider variety of applications.

In addition, due to the small sizes of the vibrating elements that canbe achieved using semiconductor processing techniques, the compact sizeof the ASIC layer, and the low driving voltages required for driving thevibrating elements, piezoelectric transducer devices suitable forhigh-precision medical diagnostic and therapeutic applications can bedeveloped using the designs disclosed in this specification. Forexample, the low voltage, low heat, small size of the transducer devicescan make it safer and/or more comfortable for usage in contact with apatient's skin or inside a patient's body. In addition, dynamic imagingof small, delicate, and hard-to-access areas in a patient's body (e.g.,eyes, blood vessels, or brain) can be enabled by the transducer designsdisclosed in this specification. Also, providing an ASIC layer with anarray of driving circuitry that corresponds to an array of vibratingelements with vertical connections to the vibrating elements can enablethe vibrating elements to closely packed, which can improve imagequality. In addition, the resonance frequencies and impedance of thecurved vibrating elements can be controlled in the design andmanufacturing process of the piezoelectric transducer, for example, byvarying the size, shape, and thickness of the curved piezoelectricelements. Therefore, applications that require different operatingfrequency ranges, and different types of propagation media (e.g.,different impedances) can all be accommodated.

In some implementations, the curved piezoelectric element can be formedby depositing particles of a piezoelectric material (e.g., sputtering)on a curved profile-transferring substrate. Alternatively, the sputteredpiezoelectric material can form a flat piezoelectric element. The curvedor flat piezoelectric element formed by deposition has columnar grainstructures that are aligned and perpendicularly oriented relative to thecurved or flat surfaces of the piezoelectric element and a naturalas-deposited poling direction pointing in the direction of the columnargrains. Such aligned columnar grain structure poses less internal stressduring operation, leading to longer lifetime of a piezoelectrictransducer formed for such piezoelectric elements.

In some implementations, the peripheral portion of the curved portion ofeach piezoelectric element is kept stationary during operation, while acentral portion of the curved portion flexes in response to the drivingvoltages. By placing the transition point between the flexing portionand the stationary portion of the piezoelectric element in an area ofthe piezoelectric element where the grain structures are more aligned,the piezoelectric element can better withstand the high stress occurringat the transition point, which can lead to longer lifetime of thepiezoelectric element and the piezoelectric transducer device.

The details of one or more implementations of the subject matterdescribed in this specification are set forth in the accompanyingdrawings and the description below. Other features, aspects, andadvantages of the subject matter will become apparent from thedescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1H illustrate example configurations of piezoelectrictransducer devices that include array(s) of curved vibrating elements.

FIG. 2A-2B illustrate vertical cross-sections of two examplepiezoelectric transducer devices including curved vibrating elements.

FIGS. 3A-3B are each a schematic illustration of the deflections of acurved piezoelectric element under an applied voltage or mechanicalpressure.

FIGS. 4A-4B are images showing the grain structures within a curvedpiezoelectric element formed by depositing piezoelectric materialparticles over a curved profile-transferring surface.

FIG. 5A is a schematic top view of an example micro-dome transducerarray in a piezoelectric transducer device.

FIG. 5B is a perspective view of a portion of the micro-dome transducerarray in the piezoelectric transducer device shown in FIG. 5A.

FIG. 5C is a close-up top view of the micro-dome transducer array shownin FIG. 5A.

FIG. 6 is a schematic illustration of the circuitry functions that canbe implemented in an integrated ASIC layer below the array of vibratingelements in a piezoelectric transducer device.

Many of the layers and features are exaggerated to better show theprocess steps and results. Like reference numbers and designations inthe various drawings indicate like elements.

DETAILED DESCRIPTION

A piezoelectric ultrasonic transducer device is capable of generatinghigh frequency pressure waves in a propagation medium (e.g., air, water,tissue, bone, metal, etc.) using a piezoelectric transducer arrayvibrating in response to a high frequency time-varying driving voltage.An exposed outer surface of the vibrating transducer array can be placedclose to or in contact with the propagation medium to couple the energycarried by the vibrations of the exposed outer surface to the energycarried by the pressure waves propagating along one or more directionsin the propagation medium. An ultrasonic transducer device typicallygenerates sound waves with frequencies above the human audial range.However, in some implementations, piezoelectric transducer devices madeaccording to the descriptions in this specification can be used togenerate sound waves with frequencies within or below the human audialrange as well.

When the pressure waves encounter variations in density or elasticmodulus (or both) either within the propagation medium or at a boundarybetween media, the pressure waves are reflected. Some of the reflectedpressure waves can be captured by the exposed outer surface of thetransducer array and converted to voltage signals that are sensed by thesensing circuits of the ultrasonic transducer device. The sensed voltagesignals can be processed in conjunction with the driving voltage signalsto obtain information on the variations in density or elastic modulus(or both) within the propagation medium or at the boundary between themedia.

When the vibrations of each vibrating element in the vibratingtransducer array are individually controlled and timed with respectivetime delays and frequencies, a wave front having a desired shape, size,direction, and speed can be generated. The size and pitch of thevibrating elements, the layout of the transducer array, the drivingfrequencies, and the respective time delays and locations of thevibrating elements, can be used in conjunction with the respectivestrength and timing of the sensed voltage signals on the vibratingelements, to determine the variations in density or elastic modulus (orboth) either within the propagation medium, and to deduce the locations,sizes, shapes, and/or speeds of the objects and/or structural variationsencountered by the pressure waves in the propagation medium. The deducedinformation on the locations, size, shapes, and/or speeds of the objectsand/or structure variations in the propagation medium can be presentedon an external display device, for example, as colored or monochromaticimages. Ultrasonic transducer devices can find many applications inwhich imaging of internal structural variations within a medium ormultiple media is of interest, such as in medical diagnostics, productdefect detection, minimally-invasive surgery equipment, etc.

In this specification, piezoelectric transducer devices havingtransducer arrays formed of curved vibrating elements are disclosed. Acurved vibrating element includes a curved piezoelectric elementdisposed between a pair of curved electrodes that have respective curvedsurfaces matching the curved surfaces of the piezoelectric element. Thecurved vibrating element exhibits a larger displacement in response to agiven driving voltage and better sensing sensitivity as compared to aflat vibrating element.

In addition, the transducer array can be fabricated and integrated withan Application-Specific Integrated Circuit (ASIC) wafer usingsemiconductor fabrication processes. The ASIC wafer includes integratedcircuitry for controlling the driving and sensing functions of thetransducer device and requires only a small number of externalconnections to transmit driving signals to and collect sensed signalsfrom the large number of vibrating elements in the transducer array.

Therefore, a transducer device containing an array of curved vibratingelements and an integrated ASIC wafer as disclosed in this specificationcan be compact, light-weight, have better driving and sensingefficiencies, and require lower driving voltages as compared toconventional transducer arrays formed of flat piezoelectric films orelement made from diced bulk piezoelectric materials.

In some implementations, rather than including an ASIC wafer, thetransducer device can include an electrical interconnect layer. Thetraces for the individual transducers can be moved to the interconnectlayer so that the high density array of vibrating elements can still beachieved. The circuitry for controlling the driving and sensingfunctions for the transducer device can be located elsewhere in thedevice (e.g., external to the base or upstream to the interconnectlayer).

In some implementations, the transducer device can include an array offlat vibrating elements along with the integrated ASIC wafer. The flatvibrating elements can be formed by sputtering piezoelectric material oretching (e.g. plasma etching) bulk piezoelectric material. Sputteringpiezoelectric material or etching bulk piezoelectric material ratherthan sawing allows the vibrating elements to have a wide variety ofdesired shapes, such as shapes with rounded corners, circular shapes,pentagons, hexagons, or any other shape. This facilitates wafer levelintegration into a MEMs device. Further, the density of the transducerarray is not limited by the size of a saw blade as is the case withdiced bulk piezoelectric vibrating elements.

FIGS. 1A-1G illustrate example configurations of piezoelectrictransducer devices that include array(s) of curved vibrating elements.

In some implementations, a transducer device includes a linear orone-dimensional transducer array. The curved vibrating elements in theone-dimensional transducer array are distributed along a straight line.The vibrating outer surface of the linear transducer array can besubstantially within a plane parallel to the straight line. As shown inFIG. 1A, the transducer device 102 includes a handle portion 104. Thelinear transducer array 106 can be attached to the handle 104 at onedistal end 108 of the handle 104, where the shape of the handle 104 ismodified (e.g., widened, flattened, etc.) to accommodate the shape andsize of the transducer array 106. In this example, the vibrating outersurface of the transducer array 106 faces a forward-direction along thelong axis of the handle 104, i.e., the outer surface 105 of thesubstrate on which the array 106 is fabricated is perpendicular to thelong axis of the handle 104. In other implementations, the exposed outersurface of the linear transducer array 106 can face to the side along adirection perpendicular (or at an acute angle) to the long axis of thehandle 104. An operator of the transducer device 102 can manipulate thehandle 104 to change the direction and location of the vibrating outersurface of the linear transducer array 106 as desired (e.g., facing thearea(s) to be imaged).

The piezoelectric transducer device 102 can optionally include anintegrated ASIC wafer (not shown) below the linear array of vibratingelements 106 and inside the handle portion 104 (e.g., inside the widenedand flattened first distal end 108). Wires 110 connecting to theexternal input connections of the ASIC wafer can exit from the back endof the handle 104 and be connected to external equipment (e.g., acontrol device and/or a display device).

In some implementations, transducer devices can include two dimensionaltransducer arrays. Each two-dimensional transducer array can includemultiple curved vibrating elements distributed in a two-dimensionalarray. The area covered by the two-dimensional array can be of variousshapes, e.g., rectangular, square, circular, octagonal, hexagonal,circular, and so on. The vibrating elements in the two-dimensional arraycan be distributed on a lattice consisting of straight lines (e.g., asquare lattice or hexagonal lattice) or of more complex patterns. Thevibrating outer surface of the two-dimensional transducer array can besubstantially within a plane as well. The two-dimensional transducerarray can be attached to a handle (e.g., at one distal end of a straightcylindrical handle) to form the transducer device. The plane of thevibrating outer surface of the transducer array can face forward, e.g.,be perpendicular to, the long axis of the handle (e.g., as shown in FIG.1B), or face to the side, i.e., be parallel (or at an acute angle), tothe long axis of the handle (e.g., as shown in FIG. 1C).

An operator of the transducer device can manipulate the handle of thetransducer devices to change the facing direction and location of thevibrating outer surface of the two-dimensional transducer array asdesired (e.g., facing the area(s) to be imaged).

As shown in FIG. 1B, the piezoelectric transducer device 112 includes aforward facing hexagonal transducer array 116 attached to a handle 114at a first distal end 118. The piezoelectric transducer device 112 canoptionally include an integrated ASIC wafer (not shown) below thehexagonal array of vibrating elements and inside the handle portion 114.Wires 120 connecting to the external connections of the ASIC wafer canexit from the back (e.g., a second distal end) of the handle 114 and beconnected to external equipment (e.g., a control device and/or a displaydevice). The forward facing transducer device 112 can be used forintravascular ultrasound (IVUS) imaging, which is not feasible withconventional ultrasound imaging.

FIG. 1C shows a piezoelectric transducer device 122 that includes aside-facing square transducer array 126 attached to a handle 124 at afirst distal end 128. The piezoelectric transducer device 122 canoptionally include an integrated ASIC wafer (not shown) on the back ofthe square array of vibrating elements and inside the handle portion124. Wires 130 connecting the external connections of the ASIC wafer canexist from the back (e.g., a second distal end) of the handle 124 and beconnected to external equipment (e.g., a control device and/or displaydevice).

In some implementations, a transducer device can include aone-dimensional transducer array or a two-dimensional transducer arraythat is wrapped along a curved line or around a curved surface, suchthat the vibrating outer surface of the transducer array is a curvedline or curved surface.

For example, FIG. 1D shows an example transducer device 132 thatincludes a linear transducer array 136 that runs along a curved line andattached to a handle 134 at a first distal end 138 (e.g., an enlarged,curved, and flattened portion) of the handle 134. The transducer device132 also includes wires 140 connected to an ASIC wafer (not shown) andexiting a back end of the handle 134.

FIG. 1E shows an example transducer device 142 that includes aforward-facing linear transducer array 146 that runs around thecircumference of a circle and attached to a handle 144 at a distal end148 of the handle 144. The transducer device 142 also includes wires 150connected to an ASIC wafer (not shown) and exiting a back end of thehandle 144.

FIG. 1F shows an example transducer device 152 that includes aside-facing linear transducer array 156 that runs around thecircumference of a circle and attached to a handle 154 at a distal end158 of the handle 154. The transducer device 152 also includes wires 160connected to an ASIC wafer (not shown) and exiting a back end of thehandle 154.

In some implementations, each vibrating element of the linear transducerarrays 136, 146, and 156 shown in FIGS. 1D, 1E, and 1F can be replacedby a small two-dimensional sub-array. For example, each sub-array can bea small square transducer array. As shown in FIG. 1G, a transducerdevice 162 includes a forward-facing two-dimensional annular array 166formed of multiple square sub-arrays of vibrating elements (e.g., squaresub-arrays 168), where the forward-facing annular array 166 is attachedto a first distal end of a handle 164 of the transducer device 162. Thetransducer device 162 also includes wires 170 connected to an ASIC wafer(not shown) and exiting a back end of the handle 164.

Similarly, as shown in FIG. 1H, a transducer device 172 includes aside-facing array 176 formed of multiple square sub-arrays of vibratingelements (e.g., square sub-arrays 178), where the side-facing array 176is attached to a first distal end of a handle 174 of the transducerdevice 172. The transducer device 172 also includes wires 180 connectedto an ASIC wafer (not shown) and exiting a back end of the handle 174.

The configurations of the transducer devices shown in FIGS. 1A-1H aremerely illustrative. Different combinations of the facing direction(e.g., forward-facing, side-facing, or other facing angles) and overallshape (e.g., flat or curved, linear, polygonal, or annular) of thevibrating outer surface of entire transducer array, the positions of thetransducer array on the handle, and the layout of the vibrating elementson the transducer array are possible in various implementations of thetransducer devices.

In addition, depending on the applications (e.g., the desired operatingfrequencies, imaged area, imaging resolutions, etc.), the total numberof vibrating elements in the transducer array, the size of thetransducer array, and the size and pitch of the vibrating elements inthe transducer array can also vary. In one example, a linear arrayincludes 128 vibrating elements of 50 micron radii at a 200 micronpitch. In another example, a square array includes 16 vibrating elementsof 75 microns at a 200 micron pitch. Other example configurations aredescribed in other parts of the specification.

As disclosed in this specification, a transducer array of a transducerdevice includes multiple curved piezoelectric vibrating elements. FIGS.2A-2B illustrate two example configurations of a curved piezoelectricvibrating element.

In FIG. 2A, a convex or dome-shaped vibrating element 202 is shown. Theconvex vibrating element 202 includes a top surface 204 that is exposedand forms a portion of the vibrating outer surface of the transducerarray (e.g., along with the top surfaces of other vibrating elements inthe transducer array). The vibrating element 202 also includes a bottomsurface 206 that is attached to a top surface of a base 208.

As shown in FIG. 2A, the dome-shaped vibrating element 202 includes aconvex or dome-shaped piezoelectric element 210 disposed between areference electrode 212 and a drive electrode 214. In this example, thereference electrode 212 is disposed over the top surface (farther fromthe base 208) of the convex piezoelectric element 210, while the driveelectrode 214 is disposed below the bottom surface (closer to the base208) of the convex piezoelectric element 210. In an alternative example(not shown), the drive electrode can be disposed over the top surface ofthe convex piezoelectric element, while the reference electrode isdisposed below the bottom surface of the convex piezoelectric element.

As shown in FIG. 2A, the convex piezoelectric element 210 is a thinlayer of piezoelectric material that is of substantially the samethickness throughout. The thin layer of piezoelectric material includesa curved portion 218 surrounded by a planar portion 220. The centralcurved portion 218 curves away from the base 208 to which the vibratingelement 202 is attached. The convex piezoelectric element 210 can beformed by depositing (e.g., sputtering) piezoelectric material particlesin a uniform layer on a profile-transferring substrate that has a domeformed on a planar top surface, for example. An example piezoelectricmaterial that can be used to form the piezoelectric element 210 includesLead Zirconate Titanate (PZT).

Further as shown in FIG. 2A, the convex piezoelectric element 210 isdisposed over a drive electrode 214 on a top surface of the driveelectrode 214. The drive electrode 214 can be a thin layer of conductivematerial that has a top surface in contact with and conforms to thebottom surface of the convex piezoelectric element 210. Therefore, thedrive electrode 214 also includes a central curved portion that curvesaway from the base 208, and a planar portion adjoining (e.g.,surrounding) the central curved portion. The central curved portion ofthe drive electrode 214 and the central curved portion of the convexpiezoelectric element 210 have matching surface profiles.

In some implementations, the drive electrode 214 can be formed bydepositing a thin layer of conductive material over aprofile-transferring substrate that has a dome formed on a planar topsurface. After the layer of conductive material (i.e., the driveelectrode layer in this example) is deposited on theprofile-transferring substrate, the deposited drive electrode layer canthen serve as the profile-transferring substrate on which the thin layerof piezoelectric material for the piezoelectric element 210 can bedeposited. The conductive materials for the drive electrode layer caninclude one or more of various metals (e.g., Au, Pt, Ni, Ir, etc.),alloys (e.g., Au/Sn, Ir/TiW, Au/TiW, AuNi, etc.), metal oxides (e.g.,IrO2, NiO2, PtO2, etc.), or combinations thereof, for example.

In some implementations, suitable methods for depositing thepiezoelectric material over the drive electrode layer include, forexample, sputtering, chemical vapor deposition, physical vapordeposition, atomic layer deposition, plasma-enhanced chemical vapordeposition, and so on. Types of sputter deposition can include magnetronsputter deposition (e.g., RF sputtering), ion beam sputtering, reactivesputtering, ion assisted deposition, high target utilization sputtering,and high power impulse magnetron sputtering. The thickness of thepiezoelectric layer can be selected such that the piezoelectric element210 is sufficiently flexible to flex under the driving voltages, and yetstiff enough to transfer its vibrations to the propagation medium incontact with the exposed outer surface of 204 of the vibrating element202.

Further as shown in FIG. 2A, the reference electrode 212 is disposedover the top surface of the convex piezoelectric element 210. Thereference electrode 212 can be a thin layer of conductive material thathas its bottom surface in contact with and conforms to the top surfaceof the convex piezoelectric element 210. Therefore, the referenceelectrode 212 also includes a central curved portion that curves awayfrom the base 208, and a planar portion adjoining (e.g., surrounding)the central curved portion. The central curved portion of the referenceelectrode 212 and the central curved portion of the convex piezoelectricelement 210 have matching surface profiles.

In some implementations, the reference electrode 212 can be formed bydeposition of a thin layer of conductive material over the depositedpiezoelectric layer, for example, after the deposited piezoelectriclayer has been patterned to define the piezoelectric element 210. Theconductive materials for the reference electrode layer can include oneor more of various metals (e.g., Au, Pt, Ni, Ir, etc.), alloys (e.g.,Au/Sn, Ir/TiW, Au/TiW, AuNi, etc.), metal oxides (e.g., IrO2, NiO2,PtO2, etc.), or combinations thereof, for example.

Further as shown in FIG. 2A, in some implementations, the vibratingelement 202 can optionally include a thin membrane layer 222 below thedrive electrode 214 and in contact with the bottom surface of the driveelectrode 214. In some implementations, to form to the vibrating element202, the thin membrane layer 222 can be deposited on a domedprofile-transferring substrate first. Then, the drive electrode layercan be deposited on the top surface of the thin membrane layer 222.After the drive electrode layer is deposited, the piezoelectric layercan be deposited on the drive electrode layer. Piezoelectric layer andthe drive electrode layer can be patterned to form individual driveelectrode 214 and piezoelectric element 210, before the referenceelectrode layer is deposited on the piezoelectric element. In someimplementations, the profile transferring substrate on which themembrane layer 222 is deposited can be etched away from the bottom up toexpose the central curved portion of the bottom surface of the thinmembrane 222, such that the central curved portion can flex under anapplied voltage. In some implementations, the membrane layer 222 can bean oxide layer that functions as an etch stop for the etching process.

In some implementations, after the piezoelectric element 210 is formedon top surface of the drive electrode 214, and before the referenceelectrode layer is deposited, a layer of dielectric membrane 224 (e.g.,a SiO₂, SiN₂, or combination thereof) can be deposited on the topsurface of the piezoelectric element 210. The central portion of themembrane 224 over a central portion of the curved portion 218 of thepiezoelectric element 210 can be etched open to expose the central topsurface of the piezoelectric element 210. Then the reference electrodelayer can be deposited over the exposed top surface of the piezoelectricelement 210, such that the bottom surface of the reference electrode 212comes in contact with and conforms to the exposed top surface of thedielectric membrane 224 and the exposed top surface of the piezoelectricelement 210. The dielectric membrane 224 can serve to insulate the driveelectrode 214 from the reference electrode 212. In addition, thedielectric membrane 224 can also serve to insulate the referenceelectrode 212 from the piezoelectric element 210 in areas of thepiezoelectric element 210 that are kept stationary during operation(e.g., including the peripheral portions 238 and the planar portions 220of the piezoelectric element 210). By insulating the reference electrode212 from the piezoelectric element 210 in areas that are keptstationary, internal stress experienced by the piezoelectric element dueto the driving voltages applied in those areas can be reduced.

In some implementations, where there are only a small number ofvibrating elements in the transducer array, electrical connections tothe drive electrode and the reference electrode of each vibratingelement can be made through conductive traces that run in a same planeparallel to the top surface of the base 208. In some implementations,the reference electrode of several vibrating elements can be joined toform a common reference electrode that spans multiple vibratingelements. In some implementations, where there are many vibratingelements distributed in a small lateral area, there may not be enoughspace between individual vibrating elements to run conductive traces ina single plane parallel to the top surface of the base 208. Instead, arespective vertically-oriented electrical interconnect can be made foreach of some or all vibrating elements in the transducer array toconnect the vibrating element to the control/sensing circuitry in thebase 208. As shown in FIG. 2A, a vertically-oriented electricalinterconnect 226 is used to electrically connect the drive electrode 214to an active connection pad 228 of the control/sensing circuits 230 inthe base 208. In addition, a vertically-oriented electrical interconnect232 is used to electrically connect the reference electrode 212 to aground connection pad 234 of the control/sensing circuit 230 in the base208. In some implementations, where a common reference electrodespanning the entire transducer array is used, only one electricalconnection is needed to electrically connect the common referenceelectrode to the ground connection pad in the base 208.

In some implementations, as shown in FIG. 2A, the flexible portion ofthe vibrating element 202 is suspended above the top surface of the base208. The spacing between the top surface of the base 208 and the bottomsurface 206 of the vibrating element 202 can be created by a support236, e.g., an annular support, having a hole 241 vertically aligned withthe curved portion 218 of the vibrating element 202. The support 236 hasa bottom surface attached to the top surface of the base 208, and a topsurface attached to the bottom surface 206 of vibrating element 202. Inimplementations where no membrane 222 exists below the drive electrode214, the bottom surface of the drive electrode can serve as the bottomsurface 206 of the vibrating element 202. If the membrane 222 exists,the bottom surface of the membrane 222 can serve as the bottom surfaceof the vibrating element 202. The support 236 can also serve to isolateindividual vibrating elements on the base 208 to reduce cross-talkbetween adjacent vibrating elements. In some implementations, the heightof the support can be very small, such that only a small gap (e.g., afew microns) exists between the central vibrating portion (in otherwords, the flexible portion) of the vibrating element 202 and the topsurface of the base 208. In some implementations, the height of thesupport 236 can be thicker, e.g., 10-50 microns or more.

In the example configuration shown in FIG. 2A, the reference electrode212 is positioned over the piezoelectric element 210, while the driveelectrode 214 is positioned below the piezoelectric element 210. In analternative configuration, the reference electrode can be positionedbelow the piezoelectric element, while the reference electrode ispositioned above the piezoelectric element. The vibrating element havingthe alternative configuration can be formed in a similar manner asdescribed above, except that a lower metal layer can be used as thereference electrode layer, while the higher metal layer can be used asthe drive electrode layer. In addition, the vertically-orientedelectrical interconnects will each reach into a different metal layer inthe vibrating element.

Continuing with the example shown in FIG. 2A, in some implementations, aprotective membrane (not shown), e.g., a non-wetting coating, a PECVDoxide coating or the like, can be deposited on the top surface of thereference electrode 212. In some implementations, the protectivemembrane can be a continuous layer deposited over the entire top surfaceof the transducer array. The top surface of the protective membrane canbe exposed and free of any additional layers deposited on top. Inoperation, the exposed top surface of the optional protective membranecan be put into direct contact with the propagation medium (which couldbe the material of interest, e.g., tissue, or a coupling medium, e.g.,an ultrasound transmission gel) such that vibrations of the vibratingelements in the transducer array can be coupled into pressure waves inthe propagation medium.

The optional protective membrane can protect the vibrating element 202from dust and contamination, and possibly corrosion due to moisture orother substances in the propagation medium. In addition, a protectivemembrane may be advantageous in implementations where the driveelectrode is placed above the piezoelectric element 210 while thereference electrode is placed below the piezoelectric element 210, andwhere the protective membrane can serve as an insulation layer betweenthe drive electrode (now positioned on the top of the vibrating element)and the propagation medium (e.g., a patient's skin or tissue).

In some implementations, no protective membrane is used in the vibratingelement 202 and the top surface of the reference electrode 212 can beexposed to the propagation medium directly when in use. For example, incases where a continuous reference electrode is used to cover the entiretransducer array and the continuous reference electrode is made of aninert metal (e.g., Au or Pt) resistant to corrosion and contamination,and where the reference electrode is kept at an earth ground potential,then the continuous reference electrode can serve as the exposed outersurface of the transducer device that is used to couple vibrations ofthe vibration elements to the propagation medium.

In some implementations, as shown in FIG. 2A, the curved portion of thevibrating element 202 (including the central flexible portions of thereference electrode, piezoelectric element, and drive electrode, and anyadditional membrane layers above, below, or in between the referenceelectrode, piezoelectric element, and/or drive electrode) remain curvedin the absence of any applied voltage between the drive electrode andthe reference electrode. When a time-varying driving voltage is appliedbetween the drive electrode 214 and the reference electrode 212, thecurved portion (or a central portion thereof) of the vibrating element202 can vibrate in response to the time-varying driving voltage.

In some implementations, only the planar portion of the vibratingelement 202 is affixed to the base 208 (e.g., by the support 236), andremains stationary during operation, while the entire curved portion ofthe vibrating element 202 vibrates in response to the time-varyingdriving voltage. In some implementations, however, a peripheral portion238 of the curved portion can also be affixed to the base 203 by thesupport 236, such that the peripheral portion 238 of the curved portioncan remain stationary during operation, while only the central portion240 of the curved portion vibrates in response to the time-varyingdriving voltage. In some of the implementations where the peripheralportion 238 of the curved portion is affixed to the base, all or some ofthe planar portion of the vibrating elements can be removed.

In some implementations, the peripheral portion 238 that is affixed andkept stationary can account only a small width relative to the lateraldimension of the curved portion 218. As will be shown later in thespecification, by keeping a small peripheral portion 238 of the curvedportion 218 stationary during operation, stress at the transition pointsbetween the curved portion 218 and the planar portion 220 of thepiezoelectric element 210 can be reduced, extending the lifetime of thevibrating element 210.

In some implementations, where the peripheral portion 238 of the curvedportion 218 of the piezoelectric element 210 is affixed and keptstationary during operation, the planar portion 220 surrounding theperipheral portion 238 of the curved portion 218 can be reduced in sizeor eliminated completely in the vibrating element 202, as long as thehold on the peripheral portion 238 by the support 236 is of sufficientstrength and durability to prevent the piezoelectric element 210 fromdelaminating from the electrodes 212 and 214 during an acceptable lifetime for the piezoelectric transducer device.

FIG. 2B shows another example configuration for a vibrating element 242in which a concave piezoelectric element 250 is used. The vibratingelement 242 can be constructed similarly to the vibrating element 202,except as described below. In the example vibrating element 242, thereference electrode 252 is disposed below the bottom surface of theconcave piezoelectric element 250, while the drive electrode 254 isdisposed above the top surface of the concave piezoelectric element 250.An optional membrane layer 262 can be disposed below the bottom surfaceof the reference electrode 252. The bottom surface 246 of the vibratingelement 242 can be the bottom surface of the membrane layer 262, ifpresent, or the bottom surface of the reference electrode 252, if nomembrane layer 262 is used. In addition, a dielectric layer 264 can bedeposited over the piezoelectric element and the reference electrodelayer to insulate the drive electrode 254 from the reference electrode252 and to insulate the drive electrode 154 from the piezoelectricelement 250 in areas outside of the central area on the top surface ofthe piezoelectric element 250.

In addition, an optional protective membrane 263 can be used to coverthe top surface of the drive electrode 254. The top surface of theprotective membrane 263 can serve as the exposed top surface of thevibrating element 242 and put in contact with the propagation mediumduring operation. In some implementations, the protective membrane 263is a thin dielectric layer that serves to insulate the drive electrode254 from the propagation medium.

In addition, a vertically oriented electrical interconnect 266 canelectrically connect the drive electrode 254 to the active connectionpad 268 of the control/sensing circuitry 270 in the base 248. Avertically-oriented electrical interconnect 272 can electrically connectthe reference electrode 252 to the ground connection pad 274 of thecontrol/sensing circuitry 270 in the base 248. In some implementations,the reference electrode 252 can be a continuous conductive layer thatspans the entire transducer array in a transducer device.

Although the example shown in FIG. 2B has the reference electrode 252disposed closer to the base 248, while the drive electrode 254 isdisposed farther away from the base 248, in an alternativeimplementation, the positions of the reference electrode 252 and thedrive electrode 248 can be reversed. When the positions of the referenceelectrode 252 and the drive electrode 254 are reversed, the lengths ofthe electrical interconnects 266 and 272 can be adjusted to connect tothe appropriate electrode in the vibrating element 242.

As shown in FIG. 2B, the vibrating element 242 is similar to thevibrating element 202 shown in FIG. 2A, except that the piezoelectricelement 250, the drive electrode 254, and the reference electrode 252each include a respective curved portion that curves toward the base248. In some implementations, the concave reference electrode 252,piezoelectric element 250, and drive electrode 254 can be formed bysequentially depositing the reference electrode layer, the piezoelectriclayer, and the drive electrode layer on the top surface of aprofile-transferring substrate that has a dented portion (or inverteddome) surrounded by a planar portion.

In some implementations, similar to the vibrating element 202 shown inFIG. 2A, in addition to the planar portion 260 of the piezoelectricelement 242, the peripheral portion 278 of the curved portion 258 of thevibrating element 242 is also kept stationary during operation, whileonly the central portion 280 of the curved portion 258 vibrates inresponse to the time-varying driving voltage.

While FIGS. 2A and 2B show the electrical interconnects 226, 232, 266,272 contacting a bottom surface of electrodes 214, 212, 254, 252 in someimplementations, the electrical interconnects can contact a top surfaceof the electrodes.

In some implementations, the vibrating element 242 is suspended aboveand attached to the base 248 by a support 276 (e.g., an annular supportwith a hole 281 aligned with the curved portion 258 of the piezoelectricelement 250). Since a concave vibrating element 242 is used in theexample configuration shown in FIG. 2B, the spacing between the bottomsurface 246 of the vibrating element 242 and the top surface of the base248 should be large enough to keep the bottom of the vibrating portion280 from making contact with the top surface of the base 242 when thevibrating element 242 is experiencing a maximum displacement under adriving voltage or impinging pressure. Thus, the thickness of thesupport 276 should be selected to accommodate the maximum displacementthat is anticipated for the vibrating element 242.

Although a support 276 is used to support and suspend the vibratingelement shown in FIGS. 2A and 2B, in some implementations, a cavity canbe formed in the base (e.g., the base 208 or 248) below the flexibleportion of the vibrating element (e.g., the vibrating element 202 or242) to create the space for accommodating the displacement of thevibrating element during operation (e.g., driving and/or sensing).Alternatively, the electrical interconnect 226, 272 can be a support(e.g. an annular support) that defines the height between the vibratingelement 202, 252 and the base 208, 248 rather than supports 236, 276.

While FIGS. 2A and 2B show piezoelectric element 210, 250 as a discreteelement for an individual vibrating element, the piezoelectric elementcan be a continuous layer for multiple vibrating elements.

While FIGS. 2A and 2B show curved vibrating elements 202, 242, thevibrating elements can alternatively be flat. FIG. 3A is a schematicdiagram illustrating the deformations of the convexly curvedpiezoelectric element 302 under an applied voltage (e.g., duringdriving) and an applied mechanical pressure (e.g., during sensing).

In FIG. 3A, suppose that the piezoelectric element 302 has anas-deposited poling direction that points from the left surface of thepiezoelectric element 302 to the right surface of the piezoelectricelement 302, and that is locally perpendicular to the two surfaces ofthe piezoelectric element 302. Further suppose a drive electrode (notshown) is disposed next to the left surface of the piezoelectric element302, while a reference electrode (not shown) is disposed next to theright surface of the piezoelectric element 302. This configurationcorresponds to the configuration shown in FIG. 2A, for example. Based onthe above configuration, a positive voltage applied across thepiezoelectric element 302 between the drive electrode and the referenceelectrode can cause an electric field in the piezoelectric element 302pointing from the left surface to the right surface of the piezoelectricelement 302. In other words, the applied positive voltage causes anelectric field in the piezoelectric element 302 that is locally alignedand parallel to the poling direction in the piezoelectric element 302.As a result, the piezoelectric element 302 contracts and the curvedportion of the piezoelectric element 302 retracts to the left (shown aspiezoelectric element 302″), moving the center O of the curved vibratingelement 302 along its central axis from a resting position 304 to a newposition 308 to the left of the position 304. When the applied positivevoltage is removed, the curved portion of the piezoelectric element 302expands back to its original shape, and the center O of the curvedvibrating element 302 is restored to its original resting position 304.The vibrating of the piezoelectric element 302 can send a pressure wave310 into the propagation medium 303 on the right of the piezoelectricelement 302.

In some implementations, when a negative voltage is applied across thepiezoelectric element 302 between the drive electrode and the referenceelectrode, the piezoelectric element 302 can expand and the center O ofthe curved piezoelectric element 302 (now shown as 302′) can be movedfurther to the right of the resting position 304 to a new position 306.When the negative voltage is removed, the center O of the curvedpiezoelectric element 302 can be restored to its original restingposition 304.

In some implementations, a driving signal including alternating negativevoltage and positive voltage signals can be used to cause the vibratingelement to vibrate between a maximum positive and a maximum negativedisplacement positions (e.g., positions associated with the centerpositions 306 and 308). In some implementations, only positive voltagesare used to drive the vibrating elements, and the positive voltagesignals can be applied as pulses over a constant reference voltage. Insome implementations, it is advantageous to avoid using negative voltagesignals when driving the vibrating elements. For example, in theconfiguration shown in FIG. 3A, a negative driving voltage would inducean electric field that is antiparallel to the poling direction of thepiezoelectric element 302, which may tend to depolarize thepiezoelectric element 302 and lead to deteriorated performance of thepiezoelectric element 302 over time.

In some implementations, when the vibrating element is in a sensingmode, and no voltage is applied to the curved piezoelectric element 302,the curved piezoelectric element 302 can deform in response to anapplied mechanical pressure. For example, when pressure waves in thepropagation medium 303 are reflected back toward the vibrating elementand intercepted by the exposed surface of the vibrating element, thecurved surface of the piezoelectric element 302 can be pushed from theresting position to a position left of the resting position. The centerO of the curved piezoelectric element 302 can be moved from a restingposition 304 to a new position to the left of the resting position 304.As a result of the deformation, a voltage difference can be causedbetween the left surface and the right surface of the vibrating element302. The timing and the strength of the voltage difference can be usedto determine the variations in density and elastic modulus in thepropagation medium 303 (and hence, the position of the object orstructural variations in the propagation medium 303) that caused thereflection of the pressure wave in the propagation medium 303.

In some implementations, the same vibrating elements in the transducerarray can be used both for driving a pressure wave in the propagationmedium 303 and for sensing reflected pressure waves from the propagationmedium 303. The vibrating elements can switch between the driving andsensing mode based on control signals generated by a switching circuitin the base. In some implementations, the vibrating elements used fordriving and sensing can be separated in the transducer array, forexample, the driving vibrating elements and the sensing vibratingelements can be alternately distributed in the transducer array, andoperate synchronously.

Although FIG. 3A shows a dome-shaped or convex piezoelectric element 302having an as-deposited poling direction pointing to the right, the sameprinciples used in driving and sensing the vibrations of the dome-shapedpiezoelectric element also apply to driving and sensing the vibrationsof a dent-shaped or convex piezoelectric element.

For example, as shown in FIG. 3B, suppose the dent-shaped piezoelectricelement 322 has an as-deposited poling direction pointing from the leftsurface to the right surface of the piezoelectric element 322, a driveelectrode (not shown) is disposed on the left side of the dent-shapedpiezoelectric element 322, and a reference electrode (not shown) isdisposed on the right side of the dent-shaped piezoelectric element 322.

According to the above configuration, when a positive voltage is appliedbetween the drive electrode and the reference electrode, an electricfield can be induced in the piezoelectric element 322. The inducedelectric field is aligned and parallel to the poling direction of thepiezoelectric element 322. As a result, the piezoelectric element 322can contract (e.g., represented by the piezoelectric element 322′) andthe center O of the curved piezoelectric element 322 can be shifted anew position 326 to the right of its resting position 324. When thepositive voltage is removed, the piezoelectric element 322 is restoredto its original resting shape. The vibration of the vibrating element322 can excite a pressure wave 330 in a propagation medium 333 incontact with the exposed the concave surface of the vibrating element.

Similarly, when a reflected pressure wave exerts a mechanical pressureon the dent-shaped right surface of the vibrating element 322, thepiezoelectric element 322 can be extended to the left (e.g., representedby the piezoelectric element 322″). The center O of the piezoelectricelement 322 can be moved from its resting position 324 to a new position328 and a voltage difference can be induced between the left surface andthe right surface of the piezoelectric element 322. The timing and thestrength of the voltage difference can be used to determine thevariations in density and elastic modulus in the propagation medium 333and deduce the locations of the objects or structural variations in thepropagation medium 333 that caused the reflected pressure wave.

As set forth earlier, sputtered piezoelectric material can have a largeas-deposited poling. Some environments that are used for sputtering thepiezoelectric material include a direct current (DC) bias duringsputtering. The DC field causes the piezoelectric material to be poledin the direction of the DC field. In some implementations, theas-deposited poling direction in the deposited piezoelectric layer(e.g., sputtered PZT) can be locally perpendicular to the surface of theunderlying profile-transferring substrate, and pointing in a directionaway from the substrate surface.

If a desired poling direction in the piezoelectric element is differentfrom the as deposited poling direction, the piezoelectric membrane canbe deposited on a profile-transferring substrate and then flipped overand bonded to another substrate to obtain the desired poling direction.

While FIGS. 3A and 3B show curved piezoelectric elements 302, 322, thepiezoelectric elements can alternatively be flat.

FIG. 4A shows a Scanning Electronic Microscope (SEM) image 400 of apartial cross-section of a convex piezoelectric membrane. The image 400shows the grain structure of a sputtered PZT layer 402 deposited on adome-shaped Iridium electrode layer 404. The dome-shaped Iridiumelectrode layer 404 is suspended over a silicon substrate.

The grain structures within the PZT layer 402 are roughly columnar, andall or substantially all columnar grains are locally perpendicular tothe surface of the curved PZT layer 402. The aligned columnar PZT grainstructures shown in FIG. 4A occur when the PZT is deposited or growngradually on a curved underlayer (e.g., on the curved surface of an etchstop layer or profile-transferring substrate). The aligned columnargrain structures that are locally perpendicular to the curved surface ofthe piezoelectric membrane would not inherently occur in a bulkpiezoelectric material that is ground into a curved membrane. Nor wouldsuch grain alignment and orientation inherently occur in a curvedpiezoelectric membrane formed by injection molding.

When the grain structures in the sputtered PZT membrane are aligned andlocally perpendicular to the curved surface of the PZT membrane, areduced amount of localized internal stress occur within the membraneduring vibration of the membrane as compared to a membrane that hasrandomly oriented grain structures (e.g., such as in a membrane formedfrom bulk PZT or injection molding). With the reduced amount oflocalized internal stress, the PZT membrane having aligned columnargrains such as that shown in FIG. 4A can enjoy a longer usable life thanthe membranes produced using other conventional methods (e.g., bygrinding or by injection molding).

FIG. 4B is an enlarged SEM image 410 of the curved sputtered PZTmembrane 402 near a transitional region 412 between a curved portion anda planar portion of the sputtered PZT membrane 402. The grain structuresof the sputtered PZT in the transitional region 412 are squeezed towardthe center of the transitional region 412. The transitional region 412is less sturdy during vibration than other more homogeneous regions inthe sputtered PZT membrane 402 where the grain structures are moreparallel and aligned.

Normally, a larger amount of stress is created in the piezoelectricmembrane near the boundary between the vibration portion and thestationary portion of the piezoelectric membrane during operation. As aresult, if the transition point between the planar portion and thecurved portion of the piezoelectric membrane 402 were placed exactly atthe transition point between the vibrating portion and the stationaryportion of the piezoelectric membrane 402 (e.g., when the piezoelectricmembrane 402 is affixed to the base only at the planar portion and notat the curved portion of the piezoelectric membrane 402), thepiezoelectric membrane 402 can be prone to breakage after prolongedusage.

In some implementations, a peripheral portion of the curved portion ofthe PZT membrane 402 can be affixed to the base, and kept stationaryduring operation (e.g., as shown in the configurations in FIGS. 2A-2B).In other words, the weaker transition point between the curved portionand the planar portion in the piezoelectric membrane 402 is moved awayfrom the transition point between the vibrating portion and thestationary portion of the piezoelectric membrane 402. Instead, thetransition point between the vibrating portion and the stationaryportion is moved to a stronger, more aligned and homogeneous portion ofthe piezoelectric membrane 402 (e.g., in a curved portion of thepiezoelectric membrane 402). By shifting the high stress region to thestronger region in the piezoelectric membrane 402, the piezoelectricmembrane 402 is less prone to breakage due to the internal stress causedduring vibration of the piezoelectric membrane 402.

While FIGS. 4A and 4B show a curved piezoelectric membrane including asputtered piezoelectric layer 402 and electrode layer 404, thepiezoelectric membrane can alternatively be flat. The flat sputteredpiezoelectric layer has the same benefits of the columnar grainstructure as the curved sputtered piezoelectric layer. The columnargrain structure is locally perpendicular to the surface of the flatpiezoelectric layer. Because the flat piezoelectric layer is planar, itdoes not have a transition region between a curved portion and a planarportion. Thus, the flat piezoelectric membrane has more flexibility inwhere it can be attached to the base.

As shown above, FIGS. 2A-4B illustrate example designs and structures ofindividual vibrating elements that can be used in a transducer array ofa transducer device. A piezoelectric transducer device can include oneor more transducer arrays that each includes multiple vibratingelements. FIG. 5A illustrate an example hexagonal transducer array 502that can be included in an ultrasonic transducer device 504. In anexample implementation, approximately 1000 vibrating elements 506 with asize of 25 microns can be distributed within a hexagonal array 502 witha lateral dimension of approximately 3 mm. In another exampleimplementation, approximately 400 vibrating elements with a size of 60microns can be distributed within a hexagonal array with a lateraldimension of approximately 3 mm. In yet another example implementation,approximately 378 vibrating elements with a size of 60 microns can bedistributed within a square array with a lateral dimension of 2.13 mm.The above dimensions are illustrative, other dimensions of the vibratingelements and array sizes, pitches, and layouts are possible. The aboveexample dimensions can be suitable for ultrasonic devices forintravascular diagnostic or therapeutic uses, for example.

FIG. 5B shows a perspective view of a cross-section of the transducerarray 502 in the transducer device 504 shown in FIG. 5A. As shown inFIG. 5B, the top surface 508 of the transducer array 504 is exposed, andcan be put into contact with a propagation medium. When the top surfaceof the vibrating elements 506 in the transducer array 502 vibrates inresponse to driving voltages, the vibration of the top surface of thevibrating elements 506 can cause a pressure wave to be generated in thepropagation medium.

As shown in FIG. 5B, the vibrating elements 506 in the transducer array502 includes a vibrating portion 510 suspended above a cavity or hole512 created by an annular support (e.g., the annular metal seal 514)attached to the vibrating portion 510 and top surface of the base 516.The annular metal seals 514 are attached to the planar portion of thevibrating elements 506 as well as the peripheral portion of the curvedportion of the vibrating elements 506. In addition, the annular metalseal 514 supporting each vibrating element 506 is isolated from theannular metal seals supporting other vibrating elements 506, forexample, by an air or vacuum gap 518. In some implementations, thethickness and width of the annular seal 514 are chosen such that theseal 514 substantially prevents vibrational crosstalk between adjacentvibrating elements 506 in the transducer array 502. In someimplementations, a backing layer can be put within the cavity 512 abovethe top surface of the base 516 to absorb the energy transferred to thevibrating element 506 from reflected pressure waves, to reduce noise inthe sensed signals and the echoes within the cavity 512 caused by thevibrations of the vibrating element 506.

While FIG. 5B show curved vibrating elements 506, the vibrating elementscan alternatively be flat.

In some implementations, the annular metal seal 514 can be created by aeutectic bonding process. For example, the vibrating elements 506 andthe base 516 of the transducer array 502 can be prepared in separateprocesses. Then, metals that can be bonded using a eutectic bondingtechnique can be plated on the bottom surface of the array of vibratingelements 506 and the top surface of the base 516, respectively, and atcorresponding locations. Then the array of vibrating elements 506 can bebonded to the base 516 at the locations where the metals are plated. Thebonded metals can form the annular seal 514 that attach the vibratingelements 506 to the base 516. In some implementations, other suitablematerials (e.g., ceramics) may be used to form the seals.

In some implementations, the base 516 includes an ASIC layer forproviding control signals to the vibrating elements and for registering(e.g., compress, package, and send out) the sensed signals received fromthe vibrating elements. Therefore, in addition to the annular seals 514,electrical connection pads and connection bumps can be plated on the topsurface of the base 516 that leads to the electrical ground and theactive elements of the driving/sensing circuits in the ASIC layer in thebase 516. Corresponding electrical connection pads and connection bumpscan be plated on the bottom surface of the transducer array 502, wherethe connection pads are electrically connected to individual driveelectrodes of the vibrating elements 506 in the transducer array 502.The ASIC layer in the base 516 includes an array of driving or sensingintegrated circuits that correspond to the transducer array 502, e.g.,one circuit for each vibrating element 506. Each circuit in the array inthe ASIC layer can have the same circuit structure and function. Thereis also an array of vertically-extending electrical interconnectscorresponding to the transducer array 502 and corresponding to the arrayof circuits in the ASIC layer, e.g., at least one vertically-extendingelectrical interconnect for each vibrating element 506 to connect to thedrive electrode of the vibrating element 506. If there is a commonreference electrode for the transducer array 502, than at least onevertically extending electrical connection connect the ASIC layer in thebase 516 to the reference electrode. There can be an array of verticallyextending electrical connection for the reference electrode, althoughthere can be fewer vertically extending electrical connection that forthe drive electrodes. If there is a reference electrode for eachvibrating element, then there would be a vertically extending electricalconnection for each reference electrode, i.e., two vertically extendingelectrical connections for each vibrating element, one for the driveelectrode and one for the reference electrode.

The vertically-extending electrical interconnects can be provided by theannular seals 514, or other conductive elements such as a verticallyoriented electrical interconnect 232 (see FIG. 2A) that connects to abond pad 520. Thus, each vibrating element 506 can be connected to anassociated circuit in the ASIC by an associated vertically-extendinginterconnect. In this configuration, the leads from the vibratingelements do not require significant space on the surface of thetransducer array. Consequently the vibrating elements 506 can be moreclosely packed compared to a system with leads that extend laterally,and the closer packing can improve image quality.

When the bottom surface of the transducer array 502 and the top surfaceof the base 516 are bonded (e.g., by a eutectic bonding process) at themetal annular seals 514, the electrical connections bumps plated on thebottom surface of the transducer array 502 can be bonded (e.g., by theeutectic bonding process) to the electrical connection bumps plated onthe top surface of the base 516 to form the vertically-orientedelectrical interconnects that electrically connect the individual driveelectrodes in the transducer array 502 to their respective drivingand/or sensing circuits in the ASIC layer in the base 516. In addition,in some implementations, the ground electrode can be a shared commonground electrode, and a single vertically-oriented interconnect can bemade between the ground electrode and the electrical ground in the ASIClayer in the base 516. The single vertically-oriented interconnect forthe reference electrode can also be formed using the eutectic bondingprocess, for example.

FIG. 5C shows an enlarged top view of the transducer array 502 thatshows the flexible portions of the vibrating elements 506, the annularseals 514 supporting the flexible portions, and the electricalconnection pads 520 to the driving electrodes. In some implementations,the drive electrodes are located at the bottom of the vibrating elements506, and the electrical connections to the drive electrode can godirectly from the electrical connection pads in the drive electrodelayer vertically down to a corresponding connection pads in the topsurface of the base 516. In some implementations, the drive electrodesare located at the top of the vibrating elements 506, and the electricalconnections to the drive electrodes can go from the electricalconnection pads in the drive electrode layer vertically down, throughrespective openings the piezoelectric layer and the reference electrodelayer, to the electrical connection pads on the top surface of the base516.

As set forth earlier in the specification, the sizes (e.g., radii) ofthe vibrating portions of the vibrating elements 506 and the spacing (orpitch) of the vibrating elements 506 in the transducer array 502 can beselected based on a desired imaging resolution of the ultrasonictransducer device 504. The smaller the size of the vibrating elementsand the spacing/pitch between the vibrating elements, the better theresolution of the ultrasonic transducer is. In some implementations, thesizes (e.g., radii) of the flexible portions of the vibrating elements506 can range between 20 microns to 70 microns. The size of the array ofthe transducer device can be selected based on the desired imaging areaand the desired size of the transducer device 504. For example, for anintravascular application, the size of the array can be made smallerthan 2 mm in at least two orthogonal directions.

In some implementations, the height of the curved vibrating element(e.g., the height of the dome-shaped or the dent-shaped piezoelectricelement) in the transducer array can be chosen based on a desiredoperation range or resonance frequency and suitable impedance formatching to the impedance of an anticipated propagation medium. Forexample, for an ultrasonic transducer, the resonance frequency can rangefrom 20 KHz to 30 MHz. For medical uses, the resonance frequenciestypically range from 1 MHz to 15 MHz. The thickness of the piezoelectricelement in the vibrating element 506 can range from 3 microns to 6microns, for example. The height of the curved piezoelectric element canbe 1.5 microns to 10 microns, for example. The resonance frequencies andimpedance of the transducer array are highly tunable to suit the needsin various applications. In an example implementation, for a dome-shapedvibrating element having a radius of 50 microns and the dome angle of 25degrees, the resonance frequency is approximately 14.3 MHz, and theelectrical impedance is approximately 1.2 Kilo-Ohm at 15 MHz. Thedisplacement is approximated 82 Angstrom per volt.

In some implementations, the driving voltages of a micro-dome/denttransducer array can be 5-10 volts to achieve comparable signal strengthas that achievable using a conventional ultrasonic transducer driven at100-200 volts or more. For example, an ultrasonic transducer devicebased on the micro-dome/dent array can have a driving efficiency of upto 3 MPa/V at a 15 MHz driving frequency. As a receiver, an ultrasonictransducer device based on a micro-dome array can have a sensitivity ofup to 0.5 μV/Pa at a driving frequency of 15 MHz. In addition, themicro-dome/dent based transducer arrays can have up to 50% (6 dB)insertion loss as compared to a 20 dB loss in other conventionaltransducer technologies.

In some implementations, based on the semiconductor fabricationprocesses described in this specification, each vibrating element in themicro-dome/dent transducer array can be made very small compared to thewavelength of the ultrasonic waves that the transducer array cangenerate. In addition, the pitch between adjacent vibrating elements isnot limited by the size of the dicing blade used in making vibratingelements from a bulk piezoelectric material. Because the transducerarray can be made with small vibrating elements and have a pitch of lessthan half of the driving wavelength, each vibrating element can act as apoint source for emitting waves with uniform circular wave fronts. Thus,the entire vibrating transducer array can be used to form a beam of adesired wave front direction, focus, and shape, without any unwantedside lobes. In addition, the omni-directional radiation pattern of thevibrating elements also makes a larger acceptance angle of thetransducer device due to the radiation patterns of the vibratingelements located at the edges of the transducer array.

Although as set forth earlier, it is possible to have conductive tracesleading to the individual vibrating elements in a transducer arraywithin the same plane as the drive electrodes of the vibrating elements,such traces are possible when a small number of vibrating elementsexists in the transducer array (e.g. 4×4 array) and the spacing betweenthe vibrating elements is sufficiently large (e.g. 20-30 microns). Whena high resolution, compact transducer device is desired, an integratedASIC layer positioned directly below the transducer array can beimplemented, where vertically-oriented electrical interconnects can beformed between the drive electrodes in the transducer array and theintegrated ASIC layer. A high resolution, compact transducer device canhave, for example, more than 200 vibrating elements, such as 1000 ormore vibrating elements; a pitch of less than about 200 microns, such asbetween 100-200 microns, 65 microns or less, or 30 microns or less;and/or a resolution of about less than 0.25 mm, such as 0.1 mm or less,or 0.06 mm or less.

For example, when a large number (e.g., 1000) of vibrating elements ispacked tightly within a small area (e.g., within a circle of 3 mmradius), there is not enough space to run individual traces from thedrive electrodes within the same plane as the drive electrodes.

Instead, vertically-oriented electrical connections can be made from asmall electrical connection pad for each drive electrode to anintegrated ASIC layer in the base below the transducer array. Theintegrated ASIC layer can have multiple layers of the circuitry madefrom NMOS transistors. The design of the ASIC layers can accommodate alarge number of individual controlled outputs (e.g., 1000-2000) to besent to respective drive electrodes in the transducer array, using only100-200 external input connections. The same external input connectionscan also be used as output connections to send sensed voltage signals toan external imaging device.

FIG. 6 shows example functions that can be implemented in an integratedASIC layer in the base of transducer device 602 including amicro-dome/dent transducer array. The integrated ASIC layer cansignificantly reduce the number in external connections needed to usethe transducer device. The fewer external connections lead to reducedoverall size of the transducer device. In addition, much data processingcan be performed on board within the ASIC layer of the transducerdevice, thus the external equipment needed to process the signals neededto drive the transducer array and analyze the signals received from thetransducer array can also be reduced or simplified.

As show in FIG. 6, in some implementations, compounded control signalscan be provided to the ASIC layer through the small number of externalinput connections of the ASIC layer. The compound control signals can bede-serialized by a de-serializer 604. The de-serialized control signalscan be de-multiplexed by a de-multiplexer 606, and respective timingcontrols for the de-multiplexed signals can be added to thede-multiplexed signals. A digital to analog converter 608 can beimplemented in the ASIC layer to convert the digital control signals todriving voltage signals for the individual vibrating elements in thetransducer array. Respective time delays can be added to the individualdrive voltage signal by a programmable time-delay controller 610 tocreate the desired beam shape, focus, and direction. A switch 612 can beimplemented to switch the operation of the transducer array between thedriving mode and the receiving mode. When the transducer device 602 isin a receiving mode, the received voltages signals can be converted todigital signals using an analog to digital converter 614. The digitalsignals are compressed by a data compression unit 616 and multiplexed bya multiplexer 618. The multiplexed signals can be serialized by aserializer 620 and sent back to external processing equipment throughthe external output connections (e.g., the same external connectionsused to receive input during the driving mode) of the ASIC layer. Theabove functions are merely illustrative for the functions that can beimplemented using an integrated ASIC layer. More or fewer functions canbe included in the ASIC layer in various implementations. In addition,exact implementations, the ASIC layer can be application dependent, anddependent on the size, shape, and layout of the transducer array.

The use of terminology such as “front,” “back,” “top,” “bottom,” “left,”“right,” “over,” “above,” and “below” throughout the specification andclaims is for describing the relative positions of various components ofthe system(s) and relative positions of various parts of the variouscomponents described herein. Similarly, the use of any horizontal orvertical terms throughout the specification and claims is for describingthe relative orientations of various components of the system(s) and therelative orientations of various parts of the various componentsdescribed herein. Except where a relative orientation or position setforth below is explicitly stated in the description for a particularcomponent, system, or device, the use of such terminology does not implyany particular positions or orientations of the system, device,component or part(s) thereof, relative to (1) the direction of theEarth's gravitational force, (2) the Earth ground surface or groundplane, (3) a direction that the system(s), device(s), or particularcomponent(s) thereof may have in actual manufacturing, usage, ortransportation; or (4) a surface that the system(s), device(s), orparticular component(s) thereof may be disposed on during actualmanufacturing, usage, or transportation.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the inventions. Forexample, some processing steps may be carried out in a different order,modified, or omitted. The layout and configuration of the vibratingelements, electrodes, and electrical connections, may be varied.

1. A device for generating pressure waves in a medium, comprising: ahandle portion having a first distal end; and a vibrating portionattached to the handle portion in proximity to the first distal end ofthe handle portion, the vibrating portion comprising: a plurality ofvibrating elements, the plurality of vibrating elements sharing a commonreference electrode, each vibrating element comprising a respectivedrive electrode and a respective piezoelectric element disposed betweenthe respective drive electrode and the common reference electrode; and abase supporting the plurality of vibrating elements on a first surfaceof the base and including integrated circuitry electrically connected tothe drive electrodes and the common reference electrode of the pluralityof vibrating elements, wherein each piezoelectric element includes arespective flexible portion and a respective stationary portionconnected to a respective flexible portion, wherein the flexible portionof each piezoelectric element is suspended over the first surface and iscurved in a first direction relative to the first surface of the base inthe absence of a driving voltage applied between the respective driveelectrode of the vibrating element and the common reference electrode,wherein the flexible portion of each piezoelectric element deflects inresponse to a drive voltage applied between the common referenceelectrode and the respective drive electrode between which thepiezoelectric element is disposed, wherein the device has an exposedouter surface facing a direction pointing from the base toward theplurality of vibrating elements, and the exposed outer surface includesrespective outer surfaces of the plurality of vibrating elements or anouter surface of a flexible film covering the outer surfaces of theplurality of vibrating elements, and wherein the exposed outer surfaceof the device deflects in conformity with the deflection of the flexibleportions of the respective piezoelectric elements of the plurality ofvibrating elements.
 2. The device of claim 1, wherein the plurality ofvibrating elements form a linear array of vibrating elements.
 3. Thedevice of claim 1, wherein the plurality of vibrating elements form atwo dimensional array of vibrating elements.
 4. The device of claim 3,wherein the two dimensional array is a polygonal array.
 5. The device ofclaim 3, wherein the circuitry in the base is substantially within anarea directly below the two dimensional array of vibrating elements. 6.The device of claim 1, wherein each vibrating element is suspended abovethe first surface of the base by a respective support and the respectiveflexible portion of the piezoelectric element is suspended over thecentral portion of the respective support.
 7. The device of claim 6,wherein the respective support of each vibrating element is a metal ringthat has a first side attached to the first surface of the base, and asecond side attached to the respective stationary portion of one of thedrive electrode and reference electrode that is positioned closer to thefirst surface of the base.
 8. The device of claim 6, wherein therespective support is a metal ring formed by eutectically bonding thefirst surface of the base to the respective stationary portion of one ofthe drive electrode and reference electrode that is positioned closer tothe first surface of the base.
 9. The device of claim 1, wherein thebase has a respective cavity formed in the first surface below eachvibrating element, and the flexible portion of the piezoelectric elementis suspended over the respective cavity.
 10. The device of claim 1,wherein the respective drive electrode of each vibrating element ispositioned closer to the first surface of the base than the commonreference electrode is.
 11. The device of claim 10, wherein therespective flexible portion of each piezoelectric element is curved awayfrom the first surface of the base.
 12. The device of claim 10, whereinthe respective flexible portion of each piezoelectric element is curvedtoward the first surface of the base.
 13. The device of claim 10,wherein the common reference electrode is maintained at earth groundpotential during operation.
 14. The device of claim 1, wherein eachpiezoelectric element includes a respective curved portion surrounded bya respective planar portion, and wherein the respective planar portionand a peripheral portion of the respective curved portion of thepiezoelectric element form the respective stationary portion of thepiezoelectric element.
 15. The device of claim 14, wherein theperipheral portion of the respective curved portion of the piezoelectricelement has a width of at least 0.5 microns.
 16. The device of claim 14,wherein the peripheral portion of the respective curved portion of eachpiezoelectric element has a width of 0.5 to 10 microns.
 17. The deviceof claim 14, wherein the respective curved portion of each piezoelectricelement has thickness between 3 to 6 microns.
 18. The device of claim14, wherein the respective curved portion of each piezoelectric elementhas a height between 1.5 to 10 microns.
 19. The device of claim 14,wherein the respective curved portion of each piezoelectric element hasa lateral dimension of 15 to 80 microns in a plane perpendicular to thefirst direction.
 20. The device of claim 1, wherein grain structures ofeach piezoelectric element are columnar and substantially aligned, andall or substantially all of the columnar grains are locallyperpendicular to a surface of the piezoelectric element spanning boththe flexible portion and the stationary portion of the piezoelectricelement.
 21. The device of claim 1, wherein the handle portion has alarger size in a length dimension than in a width dimensionperpendicular to the width dimension.
 22. The device of claim 21,wherein the plurality of vibrating elements are distributed in a firstplane substantially perpendicular to the length dimension of the handleportion.
 23. The device of claim 21, wherein the plurality of vibratingelements are distributed in a first plane substantially parallel to thelength dimension of the handle portion.
 24. The device of claim 21,wherein the plurality of vibrating elements are distributed in a planesubstantially perpendicular to the length dimension of the handleportion, and within an annular area.
 25. The device of claim 21, whereinthe plurality of vibrating elements are distributed on a plurality ofplanes that are parallel to the length dimension of the handle portionand are at an equal distance to a common axis.
 26. The device of claim1, wherein the integrated circuitry in the base includes drivingcircuitry for individually driving each of the plurality of vibratingelements.
 27. The device of claim 26, wherein the driving circuitry isconfigured to drive one or more of the plurality of vibrating elementsat a frequency between 20 kHz and 30 MHz.
 28. The device of claim 26,wherein the driving circuitry is configured to drive one or more of theplurality of vibrating elements at a frequency between 1 MHz and 15 MHz.29. The device of claim 26, wherein the driving circuitry is configuredto drive one or more of the plurality of vibrating elements at a drivingvoltage at or below 10 volts.
 30. The device of claim 1, wherein theintegrated circuitry in the base includes sensing circuitry forindividually sensing pressure changes on the respective piezoelectricelements of the plurality of vibrating elements that have been appliedto the respective piezoelectric elements through the exposed outersurface of the device.
 31. The device of claim 1, wherein the device hasa maximum size of less than 2 mm in each of at least two orthogonaldirections.
 32. A device for generating pressure waves in a medium,comprising: a handle portion having a first distal end; and a vibratingportion attached to the handle portion in proximity to the first distalend of the handle portion, the vibrating portion comprising: a pluralityof vibrating elements, the plurality of vibrating elements sharing acommon reference electrode, each vibrating element comprising arespective drive electrode and a respective piezoelectric elementdisposed between the respective drive electrode and the common referenceelectrode; and a base supporting the plurality of vibrating elements ona first surface of the base and including integrated circuitryelectrically connected to the drive electrodes and the common referenceelectrode of the plurality of vibrating elements by vertically extendingelectrical connections, wherein each piezoelectric element includes arespective flexible portion and a respective stationary portionconnected to a respective flexible portion, wherein the flexible portionof each piezoelectric element is suspended over the first surface,wherein the flexible portion of each piezoelectric element deflects inresponse to a drive voltage applied between the common referenceelectrode and the respective drive electrode between which thepiezoelectric element is disposed, wherein the device has an exposedouter surface facing a direction pointing from the base toward theplurality of vibrating elements, and the exposed outer surface includesrespective outer surfaces of the plurality of vibrating elements or anouter surface of a flexible film covering the outer surfaces of theplurality of vibrating elements, and wherein the exposed outer surfaceof the device deflects in conformity with the deflection of the flexibleportions of the respective piezoelectric elements of the plurality ofvibrating elements.
 33. The device of claim 32, wherein the vibratingelements are flat.
 34. The device of claim 32, wherein eachpiezoelectric element comprises a sputtered piezoelectric layer having acolumnar grain structure.
 35. The device of claim 34, wherein thecolumnar grain structure is perpendicular to a surface of the sputteredpiezoelectric layer.